1. Field of the Invention
The present invention generally relates to methods for controlling tracker bandwidth and tracking on-time errors for improving tracker performance in wireless communication systems.
2. Description of Related Art
In spread spectrum wireless communication systems such as a Universal Mobile Telecommunication System (UMTS), base stations are not closely synchronized. Although mobile stations attempt to lock the base station carrier frequency, the operation is imperfect such that a frequency offset is present between a mobile station and a corresponding base station. The frequency offset becomes more obvious when the mobile station is in a handoff mode (a handoff mode is where a mobile station talks to two unsynchronized base stations) or the mobile station moves at a higher speed, which causes higher Doppler frequency. Without performing some type of timing error correction, an on-time error in a base station receiver can increase with time due to this aforementioned frequency offset. An on-time error, also known as a path offset error, is a timing error that represents a time difference between a detected transmission path, or a finger, and the actual transmission path.
To combat and correct these errors, base station receivers employ what is called a tracker. A tracker is commonly used in spread-spectrum wireless communication systems like UMTS and CDMA to correct path offset errors and track drifting paths. In such systems, path offsets detected by a multi-path searcher of the tracker usually has a lower resolution, for example ½ chip time, which corresponds to +/−¼ timing error from the ideal timing. This error results in nearly 1 dB loss of signal power. A tracker is therefore used to correct initial path offset errors.
In the case where a frequency offset or a Doppler frequency exists between a mobile station transmitter and base station RAKE receiver, a finger or path (e.g., detected propagation path from mobile station transmitter to base station receiver) seen by the base station receiver moves at a certain rate, known as a slew rate. The slew rate can be defined as a finger timing change rate in a chip per radio frame. The slew rate due to the frequency offset is such that an ideal path offset (i.e., ideal finger timing or ideal on-time) drifts over time. Thus, in addition to reducing initial path offset errors from the searcher outputs, a tracker should be able to track the timing drift and maintain on-time error as small as possible.